I exchanged work with someone who repairs semi's and is an experienced welder, and he built one for me. Large lock box up top and 12" shelf below. This I can say. You will never regret getting a box for your tools on your trailer. You will lose gas mileage for sure, but having the ability to lock things in there and not have to move them in and out of storage each day is priceless. I used to never have the tool I needed when I needed it because I hadn't loaded it in the morning. Now all of my hand tools, ladder, tarps, gas, oil, shrub pruners, and more go and stay in the locked box on my trailer. I doubt it will be cheap, but do it.

I don't see the need for tools on a trailer. I keep a small tool box behind the front seat in the truck. A couple reason...it is out of sight and when I unhitch from trailer I still have tools. My box of tools comes down to the exact tools needed and NO extra stuff. So I don't have a full set of socket for example. You keep all the real tools in the shop locked up tight.

Oh, I have tools behind my seat, and on my back seat in a box, and in my tool box in my bed. They are all neatly organized and in good places, but if you're doing 60-70 yards a week plus landscape projects, you need all kinds of tools at some point.

I went to an Isuzu Npr about two years ago. I put two eight foot tool boxes stacked on top of each other on one side of the bed. One of the boxes holds two weedeaters. Before when I pulled a trailer, I had a camper top on my truck that allowed me to keep all I ever needed and keep it secured.

I have a tool box welded on the front of my primary mowing trailer. Its about 36"x20"x18" (WxDxH) and stores everything that I may need, that goes with the on-board mowers or trailer. I have a box of spare belts, various tools, wheel and jack chocks, and a half gallon of 2-stroke oil with a measuring cup in there all the time.
I also put my jacket on top of everything in the spring and fall when I start to get hot at about 10 am. That way I don't have to take up a seat in my pickup with a big carhartt.

I plan on getting another here soon for my new trailer that is coming in this week!

I have a tool box on every truck and every trailer. You just don't know what truck will be with what trailer and so have duplicate works for me. Nothing more fustrating than being somewhere and needing a wrench or socket set.